1. Field of the Invention
The present invention relates to a solid fuel burner for burning solid fuel by transporting the solid fuel using gas-flow, and particularly to a solid fuel burner suitable for pulverizing, transporting using gas-flow and then suspension-burning a fuel containing much moisture and volatile matters such as wood, peat, coal or the like, and a burning method using the solid fuel burner, a combustion apparatus comprising the solid fuel burner and a method of operating the combustion apparatus.
2. Description of Prior Art
Wood, peat and coal of a low coalification rank such as blown coal and lignite which are typical thereof contain much moisture. Further, classifying fuel components into volatile matters of a component released as gas when heated, char (fixed carbon) of a component remaining as solid, ash of a component remaining as incombustible matters and moisture, these fuels contain much moisture and volatile matters and a little char. Furthermore, these fuels are low in calorific value compared to coal of a high coalification rank such as bituminous coal and anthracite, and are generally low in grindability or  pulverizability. In addition, these fuels have a property of low melting temperature of combustion ash.
Since these solid fuels contain much volatile matters, these solid fuels easily self-ignite in a storage process, a pulverizing process and a transportation process under air atmosphere, and accordingly are difficult to be handled compared to bituminous coal. In a case where these fuels are pulverized to be burned, a mixed gas of combustion exhaust gas and air reduced in the oxygen concentration is used as a transporting gas of the fuel in order to prevent these fuels from self-igniting. The combustion exhaust gas reduces the oxygen concentration to suppress oxidation reaction (burning) of the fuel and to prevent the fuel from self-burning. On the other hand, the retention heat of the combustion exhaust gas has an effect of drying the fuel by evaporating the water in the fuel.
However, when the fuel is ejected from a solid fuel burner, the oxidation reaction of the fuel transported by the transporting gas of a low oxygen concentration is limited by the oxygen concentration around the fuel. Therefore, the combustion speed is slow compared to that in a case of fuel transported by air. Since the oxidation reaction of fuel is generally activated after the fuel is mixed with air ejected from the air nozzle, the combustion speed is determined by the mixing speed with the air. Therefore, complete burning time of the fuel is longer compared to complete burning time in a case of transporting the fuel using air, and accordingly an amount  of unburned components at the exit of the combustion apparatus, that is, the furnace is increased. Further, the flame temperature is low because the combustion speed is slow. As the result, the reduction reaction of nitrogen oxides NOx to nitrogen activated in a Nox reducing zone of high temperature (about 1000° C. or higher) is difficult to be used, and accordingly the concentration of NOx at the exit of the furnace becomes higher compared to the case of transporting the fuel using air.
As the method of accelerating ignition of fuel transported by a transporting gas of low oxygen concentration, there is a method that an additional air nozzle is provided in the front end of a fuel nozzle to increase the oxygen concentration in the fuel transporting gas. For example, a solid fuel burner comprising an additional air nozzle outside the fuel nozzle is disclosed in Japanese Patent Application Laid-Open No. 10-732208.
Further, Japanese Patent Application Laid-Open No. 11-148610 discloses a solid fuel burner which accelerates mixing of fuel and air at the exit of the fuel nozzle by arranging an additional air nozzle in the center of the fuel nozzle.